1. Field of the Invention
The present invention relates to an ullage gauge and more particularly relates to an improved gauge for directly and accurately measuring the ullage of a cargo tank of a vessel such as an oil tanker. Specifically, the present invention relates to a hand ullage tape winder contained within a sealed chamber above the cargo tank but operable from the exterior of the tank to measure the ullage and to visability of the ullage tape and the interior of the cargo tank being provided by a view window within the sealed chamber.
In loading liquid cargo vessels such as oil tankers, the volume of liquid in a cargo tank is routinely determined by measuring the "ullage" of the tank after the tank is loaded. Ullage is generally defined as the amount by which a cargo tank falls short of being full and is directly related to the distance between the top of the cargo and the underside of the top of the tank. In most commercial tankers, the ullage measurement is made from a special hatch, i.e., ullage hatch, which sets atop the main hatch into the cargo tank. Once the ullage measurement is made, a standard set of ullage tables for that particular cargo tank is consulted to establish the actual volume of cargo present in the tank. The cargo tanks of vessels carrying crude oil or liquid petroleum products in bulk are commonly calibrated in cubic feet, cubic meters or barrels (42 gallons).
As the over-all size of a cargo tank increases, the accuracy of the ullage measurement becomes more and more important. For example, due to the extremely large tanks on some modern tankers, an error of one-half inch in the ullage measurement can mean an error of 5,000 gallons or more in the actual volume of cargo in that tank. The economics involved in such an error is obvious.
In a typical cargo loading operation, liquid is normally loaded into a cargo tank at a high flow rate until the tank is filled to within 5 to 6 feet of the top of the tank which is, also, normally the underside of the main deck of the vessel. The flow rate is then substantially reduced to guard against accidental overfilling which may rupture the deck. The deck is then "topped off" by slowly filling the tank to within a few inches, e.g., 6 to 24 inches, of the deck.
Tankers are presently loaded at very high loading rates often exceeding 60,000 barrels per hour. Therefore, a small lapse in timing on the part of the loading staff could lead to an over-filling. To avoid over-filling, crewmen frequently open the ullage hatch and observe the cargo level first hand while "topping off".
Up until some years after World War II, the most common means of measuring the final ullage was by a hand ullage tape reel and ullage plumb bob. Subsequently, various types of automatic ullage gauges began to appear on the market. Most of these gauges were based upon a float system with some form of spring to maintain tension and coil the tape as the cargo level rose in the tank. Usually the tape itself acted as the spring. Over the years, the number and type of gauges have proliferated and the complexity, sophistication and cost of the equipment have increased greatly. The volume of the cargo in the tank is then determined from the ullage tables using the highly reliable hand ullage or automatic ullage gauges. Despite improvements in the design and performance, the automatic ullage gauges require some maintenance and are subjected to occasional failure with or without normal maintenance. Maintenance in the cargo tanks of an oil tanker is extremely difficult because, first, the tanks must be washed and freed from all gas before they are safe for entry, and second, access to the equipment may be difficult in a large vessel.
Additionally, developments in recent years have complicated the use of the conventional hand ullage gauge in topping off cargo tanks. For example, inert gas systems which are designed to provide non-flammable gases under pressure in a cargo tank, have to be released before the plumb bob can be lowered through the ullage hatch into the tank and obviously before the cargo level can be observed first hand. Releasing the inert gas reduces the margin of safety of the system in that there is a greater risk that air might be drawn into the cargo tank, thereby creating an explosive environment within the tank. Furthermore, other safety and occupational hazards greatly reduce the desirability of releasing the inert gas and therefore, such practice is very seldom permitted.
Accordingly, since the automatic gauges presently in use are not completely reliable and since the hatch covers of modern tankers can not be opened because of the presently used inert gas systems as well as other rules and regulations concerning the occupational safety aboard modern tankers making the use of the head ullage plumb bob cumbersome, the need exists for a relatively simple ullage gauge which can provide direct observation of the cargo level and provide hand ullaging without opening the hatch cover of the cargo tank.
2. Description of the Prior Art
U.S. Pat. No. 4,099,332 issued July 11, 1978 to the present inventor discloses an ullage gauge for directly and accurately measuring the ullage of a cargo tank of a vessel such as an oil tanker without the necessity of opening the tank. This patent attempts to solve the safety hazards posed by the release of the inert gas during the "topping off" of the cargo tank. More specifically, the patent discloses a ullage gauge which comprises an indicator sword which is attached to the underside of the deck and extends downward into the cargo tank for a distance equal to the maximum expected ullage in the cargo tank. A view port is positioned through the deck adjacent the indicator sword so that the sword can be observed directly from the deck without opening the tank. The ullage gauge of this patent however, allows accurate reading of the ullage only near the top of the cargo tank, about the top 3 or 4 feet of the tank. Although such a limited measurement is generally all that is needed, with the presently used inert gas systems hindering the view of the rising level of the cargo and the high cargo loading rates commonly used, it is preferred to have a general idea of the ullage measurement over the entire cargo tank so as to better enable the prediction when the tank will be "topped off" and thus make the necessary precautions to avoid over-filling and subsequent adverse consequences.
Other patents which relate to devices for measuring changes in the volume of a liquid in a storage tank include: U.S. Pat. Nos. 4,192,187; 3,217,541; 2,852,938; 1,967,631; and 1,379,271. Each of the above patents discloses apparatus which include a tape windable onto and off a reel co-acting with a counter weight. In each of the patented apparatuses, a sight glass is provided to view the value of the cargo level measurement on the tape which is sealed within an enclosure. The apparatus disclosed in these patents rely on the balance between the counter weight and the float which is typically placed at the end of the tape as well as on the bearing system which allows the reel to rotate upon movement of the measuring tape. Accordingly, the patents disclose automatic operable gauges. Such gauges have been found to be susceptible to failure as well as requiring normal maintenance, features which have rendered automatic gauges disadvantagous in measuring the ullage in modern oil tankers for the reasons discussed above. Further, while the above prior patents disclose the addition of sight glasses to allow direct observation of the measurement from the tape, there appears to be no requirement that the sight glass allow direct observation of the rising liquid level in the tank. As discussed above, in view of the high loading rates presently used to fill the cargo tanks of oil tankers, direct observation of the rising liquid level is a substantial necessity in order to avoid over-filling.